1. Field of the Invention
The present invention relates to an automatic document feeder (ADF) and, more particularly, to a document separating mechanism included in an ADF for separating documents, or sheets, stacked on a tray one by one, feeding the separated document to a glass platen, and returning the document to another tray after illumination.
2. Discussion of the Background
A document separating mechanism for the above application may include a feed roller rotatable in an intended direction of document feed, and a separator belt turnable in a direction opposite to the direction of document feed in contact with the feed roller, as taught in, e.g., Japanese Patent Laid-Open Publication No. 3-216437, 56-99136, 1-92142, or 4-345434. However, none of the prior art mechanisms of the type using a separator belt can vary the pressure of the belt or the angle of the belt relative to the document (abutting angle hereinafter) during document feed. In practice, the ADF deals with various kinds of documents including relatively thick documents, or sheets, which are difficult to separate. Therefore, to separate even thick documents and feed them without any misfeed, it is necessary to set up a comparatively small abutting angle. In addition, to avoid misfeed and the simultaneous feed of a plurality of documents, i.e., overlapping feed, a relatively high contact pressure must be set up. A small abutting angle promotes the easy entry of the documents in the mechanism and thereby obviates misfeed. However, the small angle allows even the next document to enter the mechanism when the current document is separated. As a result, the consecutive documents rub against and contaminate each other. Further, a high contact pressure aggravates such contamination of the documents. Moreover, if the abutting angle is reduced when thin documents are dealt with, two or more of the documents are apt to enter the nip between the separator belt and the feed roller, resulting in overlapping feed.